This invention relates to an improvement in a process to recover tungsten in a usable form and in high yields from tungsten ore concentrates which may, or may not, contain scheelite.
It is the practice to process tungsten ores by one of the following four conventional methods: acid decomposition, sodium carbonate digestion in autoclaves, fusion with sodium carbonate, and caustic digestion.
In acid decomposition, hydrochloric acid is normally employed to react with the calcium tungstate to produce tungstic acid and calcium chloride. Disadvantages of this method include a high consumption of hydrochloric acid (typically from one and a half to three moles of hydrochloric acid in excess of the stoichiometric amount are required), and yields only up to about 95%. Furthermore, since removal of molybdenum as a impurity requires a basic pH, acid decomposition is in general a more expensive process than basic processes for ores containing undesirably high molybdenum levels.
Sodium carbonate digestion in an autoclave produces soluble sodium tungstate and insoluble calcium carbonate. Disadvantages of this method include a relatively high consumption of sodium carbonate, and operation at elevated temperature and pressure (typically from 180.degree. C. to 200.degree. C. at 15 atmospheres).
Fusion with sodium carbonate involves the addition of sodium carbonate and silicon dioxide to the scheelite ore concentrate and reaction to produce soluble sodium tungstate, insoluble calcium silicate and carbon dioxide gas. Disadvantages of this technique are a large consumption of sodium carbonate, very high temperatures (above 1,000.degree. C.) requiring expensive refractory materials, and the necessity for close control of the blend and temperature of reactants to obtain molten material which will flow from the reaction vessel.
U.S. Pat. No. 3,800,025 describes a lower temperature process applicable to low grade tungsten ores involving roasting in the presence of sodium carbonate or sodium phosphate and sodium chloride at between 600.degree. C. and 900.degree. C., followed by leaching and filtering. However, this process results in yields only up to about 85 percent.
Caustic soda digestion of scheelite involves the reaction of calcium tungstate and sodium hydroxide to obtain soluble sodium tungstate and insoluble calcium hydroxide according to the following reaction: CaWO.sub.4 +2NaOH Na.sub.2 WO.sub.4 +Ca(OH).sub.2. In general this technique is characterized by high caustic requirements and poor yields of extracted tungsten values. For example, in one reported experiment as much as 4.4 grams of sodium hydroxide per gram of tungsten (18 moles in excess of the stoichiometric amount required) yielded only 86.3% of tungsten from a scheelite ore concentrate. Canadian Mining and Metallurgical Bulletin, December 1966, pages 1418-1422. In U.S. Pat. No. 2,339,888 from 3 to 5 times the stoichiometric amount of NaOH is used, during digestion at elevated temperatures. The caustic may be removed from crystallized molybdenum and tungsten and recycled for use in the digestion of additional ore. In U.S. Pat. No. 3,911,077 to Martin et al. tungsten is recovered by leaching the ore concentrate in a sodium hydroxide solution having a concentration of at least 10 molar at a temperature of from 90.degree. C. to 150.degree. C. to a moisture level of from 10 to 25 percent to convert the insoluble tungstates to sodium tungstate. The reduction in moisture level is achieved by a baking step during leaching.
As set forth in U.S. Pat. No. 2,339,888, hereinbefore mentioned, the caustic liquor may be separated from crystallized sodium tungstate and sodium molybdate by filtration, and recycled after concentration for use in the digestion of additional ore. The recycled caustic which is the by-product of a hot filtering step is typically added during digestion at temperatures of about 120.degree. centigrade. The ores are charged to the hot concentrated sodium hydroxide solution and digested to form sodium tungstate.